Water absorbent resins are widely used in various applications including sanitary materials such as disposable diapers, sanitary napkins, and incontinence products for adults, and water retention agents for soil, due to their physical properties of absorbing aqueous liquids in amounts as large as several times to several hundred times the resins' own weights. Thus, water absorbent resins are produced and consumed in large quantities. These water absorbent resins (also called superabsorbent resins or water absorbent polymers) are described in, for example, the Japanese Industrial Standards (JIS) K7223-1996, and have also been introduced in many commercially available reference books.
Absorbent resins are generally provided in a powdered state, by polymerizing a hydrophilic unsaturated monomer to thereby obtain a polymer gel, and drying this. As the hydrophilic unsaturated monomer, acrylic acid (salt), particularly sodium acrylate that has been partially neutralized, is used. The polymer gel obtained by polymerizing a monomer is frequently obtained in an agglomerated form or as aggregates of polymer gel particles, and usually, the polymer gel is dried up to a solids concentration of about 95% by weight, subsequently pulverized into particles having a desired size, and classified. A water absorbent resin thus obtained is preferably subjected to a surface crosslinking step, and thus a water absorbent resin having an excellent absorption capacity under load or excellent liquid permeability is obtained. Specifically, the surface crosslinking step means a step for forming a layer having a high crosslinking density in the vicinity of the surface of the water absorbent resin, by adding a monomer or a crosslinking agent to the water absorbent resin, and allowing the mixture to react under the action of light or heat.
When a water absorbent resin is used as a sanitary material, a preferred color tone of the water absorbent resin is white, due to its clean image. Particularly, in recent years, in absorbent cores such as disposable diapers, in which the amount of the water absorbent resin is large while the amount of pulp is small, since the color of the water absorbent resin is conspicuous, a water absorbent resin is requested to have a color that is closer to the white color. However, various factors may cause the color tone of the water absorbent resin immediately after production (initial color) to turn yellow, or may cause a phenomenon (coloration over time) in which coloration (yellowing, or further coloring to brown) proceeds as a result of storage of the water absorbent resin under high temperature high humidity conditions.
In regard to this problem, various solutions have been hitherto provided. For example, Patent Literatures 1 to 7 have been suggested as technologies for preventing coloration of a water absorbent resin, which pay attention to the polymerization inhibitor. That is, a polymerization inhibitor, for example, a methoxyphenol compound, is added to the monomer so as to prevent unintended polymerization, and Patent Literature 1 discloses a method for producing a water absorbent resin in which coloration is reduced by polymerizing a methoxyphenol compound at a particular concentration. Furthermore, Patent Literatures 1 and 2 disclose the use of activated carbon in order to decrease methoxyphenols. Furthermore, in Patent Literature 3, coloration is reduced by using an N-oxyl compound or a manganese compound instead of a methoxyphenol compound. Patent Literature 4 suggests a technology of using a tocopherol compound as a polymerization inhibitor. Patent Literature 5 suggests a technology of using a hindered (methoxy)phenol compound as a polymerization inhibitor. Patent Literature 6 suggests a technology of reducing, as a polymerization inhibitor, the amount of hydroquinone which is a residual polymerization inhibitor in acrylic acid, to 0.2 ppm or less.
As another method for preventing coloration, as a technology for preventing coloration of a water absorbent resin which pays attention to the polymerization initiator, Patent Literature 7 has disclosed that an oxidizing agent such as a persulfate is used as a polymerization initiator, but the compound interacts with a methoxyphenol compound and causes coloration over time, and then suggested an initiator system which does not undergo coloration. Furthermore, Patent Literature 8 has suggested a technology for preventing coloration which uses a particular reducing agent for polymerization.
Furthermore, various coloration preventing agents for water absorbent resins have also been suggested in Patent Literatures 9 to 18, and specifically, Patent Literatures 9 to 11 suggest addition of a metal chelator; Patent Literature 12 suggests addition of an organic acid; Patent Literature 13 suggests addition of an α-hydroxycarboxylic acid; Patent Literatures 14 and 15 suggest addition of a polyvalent metal organic acid salt for surface crosslinking; Patent Literatures 16 to 18 suggest addition of a reducing agent such as a sulfite or a hypophosphite; and Patent Literature 19 suggests a technology for preventing coloration by adding an oxidation inhibitor to the water absorbent resin. Furthermore, as technologies for combined use of coloration preventing agents for the water absorbent resin, Patent Literature 20 suggests addition of an organic acid and a reducing agent to the water absorbent resin; Patent Literature 21 suggests addition of a chelating agent and a phosphorus compound to the water absorbent resin; Patent Literature 22 suggests addition of a reducing agent and an oxidation inhibitor; and Patent Literature 23 suggests addition of a chelating agent and a reducing agent to the water absorbent resin.
Furthermore, as a technology which pays attention to the cause of coloration, Patent Literature 24 has paid attention to oxygen and discloses drying or surface crosslinking in a state in which the oxygen partial pressure is decreased. Furthermore, Patent Literatures 25 and 26 have also suggested technologies which pay attention to the amount of Fe in caustic soda or polyvalent metals, which are raw materials of the water absorbent resin.
However, although the technologies of Patent Literatures 1 to 26 can reduce coloration, these are cost-consuming technologies because the number of steps is increased, and the substances to be added are increased. Furthermore, when it is intended to increase the amount of substances that are added to suppress coloration, there is a problem that the water absorption capacity decreases at least proportionally to the amount of addition, or even to a larger extent.
Meanwhile, since colored foreign substances (usually, colored particles having a size of about 0.1 mm to 1 mm) of organic substances such as a burnt water absorbent resin are present after a drying step, and unlike metallic foreign substances, a colored water absorbent resin cannot be removed with a magnet, Patent Literature 27 describes a method of selecting and removing these colored materials based on their color tones. However, there are problems with the detection of colored foreign substances, or with the cost for introducing a removing apparatus, and there is a problem that when the production output of the water absorbent resin is large, the detection of colored foreign substances may be omitted, or normal white water absorbent resins may also be removed together with the foreign substances, so that a large loss may occur.
Furthermore, as a conventional technology for drying the water absorbent resin, a method of using a ventilated band type dryer is disclosed in Patent Literatures 28 to 32 and Non-Patent Literature 1. Furthermore, a technology of using a cyclone in a method for fluidized drying in a fluidized bed is disclosed in Patent Literature 33. However, these Patent Literatures 28 to 33 neither disclose nor suggest the problems and effects of the present invention, and also, the literatures neither disclose nor suggest the passage of hot air through a filter in a ventilated band type dryer, or removal of the water absorbent resin particles or a water-containing gel thereof.